As shown in FIG. 1, an electromagnetic actuator 8 is commonly used to move a shutter 11 to control film exposure in a camera. A camera aperture 10 is normally covered with one or more shutter blades 11 to prevent film exposure to scene light. In one form of an electromagnetic actuator, shutter blade 11 is attached to a cylindrical permanent magnet 12 which is mounted for rotation about its axis 14. Permanent magnet 12 has a north pole "N" and a south pole "S". The permanent magnet is positioned between a pair of ends 16, 18 of a core 20 made of a soft magnetic material such as steel. Shutter blade 11 is maintained (rotation is prevented) in a rest state (covering aperture 10), by magnetic flux 22 coupling permanent magnet 12 to core ends 16, 18.
Referring to FIG. 2, electromagnetic actuator 8 includes an electrically conductive coil 24 which is wrapped about a portion of core 20. When electrical current is applied to coil 24, ends 16, 18 of core 20 become magnetic poles, respectively "N" and "S". These magnetic poles repel like poles on permanent magnet 12 and attract unlike magnet poles on the permanent magnet, thereby causing permanent magnet 12 and shutter blade 11 to rotate clockwise (in the direction of an arrow 25). The time it takes for shutter blade 11 to uncover aperture 10 depends on many factors. One such factor is the efficiency of the magnetic circuit (which includes permanent magnet 12, core 20 and coil 24), most notably a cross sectional area (thickness) "T" of core 20. A core with a relatively large cross-section "T" allows more electromagnetic flux to flow through the core, causing shutter 11 to open more rapidly than would occur with a core having a relatively small cross-section.
Turning to FIG. 3, when the current to coil 24 is turned off, shutter 11 is driven towards a closed position covering camera aperture 10 by the same magnetic flux 22 that holds permanent magnet 12 in the rest state (described above). There is also a magnetic flux component 26 that biases shutter 11 towards an open position. The proportion of the total magnetic flux that biases the shutter towards an open position also depends on the efficiency of the magnetic circuit, most notably the cross-sectional area (thickness) of the core. A core 20 with a relatively large cross sectional area "T" has a large proportion of magnetic flux 26 biasing shutter 11 towards an open position, thereby increasing the close time of the shutter. A core 20 with a relatively small cross sectional area "T" has only a small portion of the magnetic flux biasing shutter 11 towards an open position, thus decreasing the close time of the shutter. Thus, there is a trade-off between the time it takes to open the shutter and the time it takes to close the shutter.
The shutter open time is typically controlled by the camera electronics, which control the electrical drive time of coil 24. Often, the electrical drive time of the coil is programmed into the control electronics through software or hardware (application specific integrated circuit) and is not changeable from camera to camera as the cameras are manufactured. With a constant drive time, shutter open time may change appreciably from camera to camera due to part tolerances and magnet variation. Clearly such variation in shutter open time is a problem from a quality point of view. Further, prior art electromagnetic actuators for shutters require a different controller to be used for each of several camera models which have different shutter open times.